Semiconductor structure with buried power line and buried signal line and method for manufacturing the same

ABSTRACT

The present disclosure provides a semiconductor structure. The semiconductor structure comprises a substrate having a first top surface. An active region is surrounded by an isolation region in the substrate. A buried power line and a buried signal line are disposed within the substrate and in the active region. A first circuit layer is disposed on the first top surface of the substrate to cover the buried power line and the buried signal line. A second circuit layer is disposed on the first top surface of the substrate and separated from the first circuit layer. A cell capacitor is disposed on and electrically coupled to the first circuit layer.

TECHNICAL FIELD

The present disclosure relates to a semiconductor structure, and moreparticularly, to a semiconductor structure with a buried power line anda buried signal line and a method for manufacturing the same.

DISCUSSION OF THE BACKGROUND

A dynamic random access memory (DRAM) is a type of semiconductorarrangement for storing bits of data in separate cell capacitors withinan integrated circuit. DRAMs commonly take the form of trench capacitorDRAM cells and stacked capacitor DRAM cells. In the stack capacitor DRAMcells, the cell capacitors are formed above read/write transistors. Anadvanced method of fabricating the read/write transistors uses a buriedgate electrode, which involves a gate electrode and a word line beingbuilt in a gate trench in an active region.

Over the past few decades, as semiconductor fabrication technology hascontinued to improve, sizes of electronic devices are correspondinglyreduced. As the size of a cell transistor is reduced to a few nanometersin length, short-channel effects may occur, which may result in asignificant drop in the performance of the cell transistors.

To overcome the performance issue, there is a significant need toimprove the fabrication method of cell transistors in a semiconductorstructure.

This Discussion of the Background section is provided for backgroundinformation only. The statements in this Discussion of the Backgroundare not an admission that the subject matter disclosed in thisDiscussion of the Background section constitutes prior art to thepresent disclosure, and no part of this Discussion of the Backgroundsection may be used as an admission that any part of this application,including this Discussion of the Background section, constitutes priorart to the present disclosure.

SUMMARY

One aspect of the present disclosure provides a semiconductor structure.The semiconductor structure comprises a substrate having a first topsurface. An active region is surrounded by an isolation region in thesubstrate. A buried power line and a buried signal line are disposedwithin the substrate and in the active region. A first circuit layer isdisposed on the first top surface of the substrate to cover the buriedpower line and the buried signal line. A second circuit layer isdisposed on the first top surface of the substrate and separated fromthe first circuit layer. A cell capacitor is disposed on andelectrically coupled to the first circuit layer.

In some embodiments, the buried power line is disposed at a centralportion of the active region, and the buried signal line is disposed ata peripheral portion of the active region.

In some embodiments, the buried power line is distal to the isolationregion, and the buried signal line is proximal to the isolation region.

In some embodiments, the semiconductor structure further comprises aword line structure disposed over the cell capacitor.

In some embodiments, the semiconductor structure further comprises aninterlayer dielectric encapsulating the cell capacitor and the word linestructure.

In some embodiments, the semiconductor structure further comprises asecond conductive material within a through hole penetrating theinterlayer dielectric.

In some embodiments, the semiconductor structure further comprises a bitline structure disposed on the interlayer dielectric and over the wordline structure.

In some embodiments, the buried power line, the buried signal line andthe word line structure extend along a first direction, and the bit linestructure extends along a second direction substantially orthogonal tothe first direction.

In some embodiments, the second conductive material in the through holeextends along a third direction substantially orthogonal to the firstdirection and the second direction.

In some embodiments, the buried power line and the buried signal lineare arranged along the second direction.

In some embodiments, the word line structure and the bit line structureform a memory array, wherein the memory array has a layout of foursquare feature size (4F²).

In some embodiments, the cell capacitor is interposed between the firstcircuit layer and the word line structure, and the word line structureis interposed between the cell capacitor and the bit line structure.

Another aspect of the present disclosure provides a method offabricating a semiconductor structure. The method comprises providing asubstrate having a first top surface; forming an isolation region in thesubstrate to surround an active region; forming a recess in the activeregion; disposing a first conductive material within the recess to forma buried power line and a buried signal line; forming a first circuitlayer and a second circuit layer on the first top surface of thesubstrate, wherein the first circuit layer covers the buried power lineand the buried signal line, and the second circuit layer is separatedfrom the first circuit layer; and forming a cell capacitor over thefirst circuit layer.

In some embodiments, the method further comprises forming a word linestructure over the cell capacitor; and forming an interlayer dielectricto encapsulate the cell capacitor and the word line structure.

In some embodiments, after the formation of the interlayer dielectric, athrough hole is formed to penetrate the interlayer dielectric andpartially expose the second circuit layer.

In some embodiments, after the formation of the through hole, a secondconductive material is deposited to fill the through hole.

In some embodiments, after the formation of the second conductivematerial, a bit line structure is formed on the interlayer dielectricand over the word line structure.

In some embodiments, the bit line structure is electrically coupled tothe word line structure and the cell capacitor.

In some embodiments, the bit line structure is electrically coupled tothe word line structure, the cell capacitor and the first circuit layer.

In some embodiments, the second conductive material electricallyconnects the bit line structure to the second circuit layer.

The foregoing has outlined rather broadly the features and technicaladvantages of the present disclosure in order that the detaileddescription of the disclosure that follows may be better understood.Additional features and technical advantages of the disclosure aredescribed hereinafter, and form the subject of the claims of thedisclosure. It should be appreciated by those skilled in the art thatthe concepts and specific embodiments disclosed may be utilized as abasis for modifying or designing other structures, or processes, forcarrying out the purposes of the present disclosure. It should also berealized by those skilled in the art that such equivalent constructionsdo not depart from the spirit or scope of the disclosure as set forth inthe appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete understanding of the present disclosure may be derivedby referring to the detailed description and claims. The disclosureshould also be understood to be coupled to the figures' referencenumbers, which refer to similar elements throughout the description.

FIG. 1A is a schematic top plan view of a portion of a first memoryarray with a 6F² layout, in accordance with some embodiments of thepresent disclosure.

FIG. 1B is a schematic top plan view of a portion of a second memoryarray with a 4F² layout, in accordance with some embodiments of thepresent disclosure.

FIG. 2 is a schematic cross-sectional view of a semiconductor structure,in accordance with some embodiments of the present disclosure.

FIG. 3 is a flow diagram of a method for fabricating the semiconductorstructure in FIG. 2, in accordance with some embodiments of the presentdisclosure.

FIG. 4 to FIG. 22 are schematic cross-sectional views illustratingsequential fabrication stages according to the method in FIG. 3, inaccordance with some embodiments of the present disclosure.

FIG. 23 is a schematic top view of the semiconductor structure in FIG.22, in accordance with some embodiments of the present disclosure.

DETAILED DESCRIPTION

Embodiments, or examples, of the disclosure illustrated in the drawingsare now described using specific language. It shall be understood thatno limitation of the scope of the disclosure is hereby intended. Anyalteration or modification of the described embodiments, and any furtherapplications of principles described in this document, are to beconsidered as normally occurring to one of ordinary skill in the art towhich the disclosure relates. Reference numerals may be repeatedthroughout the embodiments, but this does not necessarily mean thatfeature(s) of one embodiment apply to another embodiment, even if theyshare the same reference numeral.

It shall be understood that, although the terms first, second, third,etc. may be used herein to describe various elements, components,regions, layers or sections, these elements, components, regions, layersor sections are not limited by these terms. Rather, these terms aremerely used to distinguish one element, component, region, layer orsection from another element, component, region, layer or section. Thus,a first element, component, region, layer or section discussed belowcould be termed a second element, component, region, layer or sectionwithout departing from the teachings of the present inventive concept.

The terminology used herein is for the purpose of describing particularexample embodiments only and is not intended to be limiting to thepresent inventive concept. As used herein, the singular forms “a,” “an”and “the” are intended to include the plural forms well, unless thecontext clearly indicates otherwise. It shall be understood that theterms “comprises” and “comprising,” when used in this specification,point out the presence of stated features, integers, steps, operations,elements, or components, but do not preclude the presence or addition ofone or more other features, integers, steps, operations, elements,components, or groups thereof.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

In silicon on insulator (SOI) technology, a floating body effect (FBE)is a phenomenon in which a threshold voltage (V_(th)) of a transistorvaries because a body of the transistor does not have a certain fixedvoltage value during operation. In other words, the threshold voltage ofthe transistor depends on the history of its biasing and carrierrecombination processes. The floating body effect causes voltagefluctuation in a body region of a SOI metal oxide semiconductor fieldeffect transistor (MOSFET), which results in detrimental effects onoperation of SOI devices. The most common of these detrimental effectsare kink effect and bipolar effect. With a channel region of the devicepartially depleted and a high drain voltage applied, an electric fieldcreated in the device causes impact ionization near a drain region.

Dynamic random access memory (DRAM) has been developed to overcome theinherent scaling limitations and to improve the cost effectiveness ofmass production. Scaling down of the DRAM has been remarkably advancedby the adoption of a trench capacitor structure and a stacked capacitorstructure. A size of a unit memory cell with one cell transistor and onecell capacitor has been reduced by evolution of a layout of a memoryarray from a six square feature size (6F²) to a four square feature size(4F²). Specifically, the minimum feature size F decreases with a newgeneration, and when the cell size is generally taken to be αF², α is acoefficient that also decreases with the advance of generation.

The main difference between the 6F² and 4F² layouts is that the 4F² cellstructure is implemented using a vertical pillar transistor (VPT), whilethe 6F² cell structure is implemented using a buried-channel-arraytransistor (BCAT). The 4F² cell is a promising architecture forcost-effective and scalable DRAM chips because of its minimized area ofcells. Due to the VPT design, the 4F² cell can be implemented in an areathat is 33% smaller than that of the 6F² cell; thus, the area of amemory cell array is reduced. The VPT device demonstrates excellentretention characteristics in static mode. The floating body effect canbe reduced by adopting the gradual junction profile even in apillar-type channel.

To avoid the floating body effect and to decrease the current leakage intransistors for low-power applications, non-silicon based materials showhigh potential when used in the 4F² cell structure because of theirintrinsically high band gap. However, high-temperature processes mightimpact electrical properties of the non-silicon based materials. Forexample, many non-silicon based materials are heat sensitive and may bedegraded by the high-temperature processes. Fabrication of a cellcapacitor generally includes several high-temperature processes.

Therefore, when the heat-sensitive non-silicon based materials are usedin the fabrication of cell transistors, processes of the cell capacitorand the cell transistor should be separated and a capacitor-firstprocess should be adopted. However, practical use is not easy sincethere is technical difficulty in that in 4F² DRAMs the cell transistormust be a vertical type. It is very difficult to decrease the area ofthe cell transistor and the cell capacitor. For example, metal routingof power lines and signal lines is challenging because of limited space.Additional metal routing should be designed in additional contact areas.

In capacitor-first processes for fabricating 4F² DRAMs, the spaceoriginally designed for the metal routing is blocked by the cellcapacitor. Therefore, in the present disclosure, the power lines andsignal lines are buried in recesses formed in the same manner as gatetrenches in the fabrication of 6F² DRAMs.

FIG. 1A is a schematic top plan view of a portion of a first memoryarray A1 with a 6F² layout, in accordance with some embodiments of thepresent disclosure. In FIG. 1A, multiple word lines WL1 are orthogonalto multiple bit lines BL1. In some embodiments, a width of each wordline WL1 and a width of each bit line BL1 are 1F, wherein the F is aminimum feature size. In some embodiments, a distance between any twoadjacent word lines WL1 and a distance between any two adjacent bitlines BL1 are also 1F. In the 6F² layout, an active region AA1 isdiagonally disposed with respect to the extending direction of the wordline WL1 or the bit line BL1. In the active region AA1, multiple memorycells (not shown) are located at the intersection of the word line WL1and the bit line BL1 and electrically coupled to the word line WL1 andthe bit line BL1. Therefore, the area of a unit memory cell in FIG. 1Ais about 3F×2F=6F², as shown by the rectangular dashed line.

FIG. 1B is a schematic top plan view of a portion of a second memoryarray A2 with a 4F² layout, in accordance with some embodiments of thepresent disclosure. In FIG. 1B, multiple word lines WL2 are orthogonalto multiple bit lines BL2. In some embodiments, a width of each wordline WL2 and a width of each bit line BL2 are 1F. In some embodiments, adistance between any two adjacent word lines WL2 and a distance betweenany two adjacent bit lines BL2 are also 1F. In the 4F² layout, an activeregion AA2 is disposed at the intersection of the word line WL2 and thebit line BL2. In addition, a unit memory cell (not shown) is located inthe active region AA2 and electrically coupled to the word line WL2 andthe bit line BL2. Therefore, the area of the unit memory cell in FIG. 1Bis about 2F×2F=4F², as shown by the square dashed line.

FIG. 2 is a schematic cross-sectional view of a semiconductor structureST1, in accordance with some embodiments of the present disclosure. Thesemiconductor structure ST1 includes a substrate 100 having a first topsurface S1. An isolation trench T1 is disposed in the substrate 100 andfilled with a first dielectric material 110. The isolation trench T1filled with the first dielectric material 110 forms an isolation regionBB in the substrate 100. An active region AA is surrounded by theisolation region BB. The active region AA is doped to form an impurityregion 114. A recess T2 is disposed in the active region AA, wherein thedepth of the recess T2 is less than that of the isolation trench T1. Theimpurity region 114 is divided by multiple recesses T2 into multipleimpurity regions 114.

An insulating liner 130 is conformally disposed within the recess T2. Afirst conductive material 140 is disposed within the recess T2 andsurrounded by the insulating liner 130. The first conductive material140 located at a central portion of the active region AA forms a buriedpower line BPL, and the first conductive material 140 located at aperipheral portion of the active region AA forms a buried signal lineBSL. The buried power line BPL and the buried signal line BSL extendalong a first direction D1. In addition, multiple buried power lines BPLand multiple buried signal lines BSL are arranged along a seconddirection D2 orthogonal to the first direction D1.

A first circuit layer 150 and a second circuit layer 152 are disposed onthe first top surface S1 of the substrate 100 and separated from eachother by a predetermined distance w1. The first circuit layer 150 coversthe buried power lines BPL and the buried signal lines BSL and iselectrically coupled to the buried power lines BPL and the buried signallines BSL. The second circuit layer 152 does not cover the buried powerlines BPL or the buried signal lines BSL.

A cell capacitor 170 is disposed on a landing pad 160 on the firstcircuit layer 150. The cell capacitor 170 is electrically coupled to thefirst circuit layer 150 via the landing pad 160. A word line structure190 is disposed on a first conductive plug 180 disposed on the cellcapacitor 170. The word line structure 190 is electrically coupled tothe cell capacitor 170 via the first conductive plug 180. The cellcapacitor 170 is interposed between the first circuit layer 150 and theword line structure 190. The word line structure 190 extends along thefirst direction D1. In some embodiments, multiple word line structures190 are arranged along the second direction D2.

A second conductive plug 200 is disposed on the word line structure 190.The stack of the landing pad 160, the cell capacitor 170, the firstconductive plug 180, the word line structure 190 and the secondconductive plug 200 extends along a third direction D3 substantiallyorthogonal to both the first direction D1 and the second direction D2.An interlayer dielectric 210 having a second top surface S2 encapsulatesthe landing pad 160, the cell capacitor 170, the first conductive plug180, the word line structure 190 and the second conductive plug 200.

A through hole T3 penetrates the interlayer dielectric 210 and exposesthe second circuit layer 152. The through hole T3 is filled with asecond conductive material 220. A bit line structure 230 is disposedover the word line structure 190. In addition, the word line structure190 is interposed between the cell capacitor 170 and the bit linestructure 230. The bit line structure 230 extends in the seconddirection D2. The second conductive material 220 deposited in thethrough hole T3 is substantially a bit line contact (BLC) electricallyconnecting the bit line structure 230 to the second circuit layer 152and to the impurity region 114. The bit line contact extends in thethird direction D3.

FIG. 3 is a flow diagram of a method 300 for fabricating thesemiconductor structure ST1 in FIG. 2, in accordance with someembodiments of the present disclosure. FIG. 4 to FIG. 22 are schematiccross-sectional views illustrating sequential fabrication stagesaccording to the method 300 in FIG. 3, in accordance with someembodiments of the present disclosure.

With reference to FIG. 4, a substrate 100 is provided according to stepS101 in FIG. 3. In some embodiments, the substrate 100 may includesingle crystal silicon substrates, compound semiconductor substratessuch as silicon germanium (SiGe) substrates, gallium arsenide (GaAs)substrates, silicon-on-insulator (SOI) substrates or other suitablesubstrates. The substrate 100 has a first top surface S1.

With reference to FIG. 5 to FIG. 9, an active area definition process isperformed on the substrate 100 according to step S103 in FIG. 3. In someembodiments, the active area definition process is a shallow trenchisolation (STI) formation process. First, referring to FIG. 5, a padoxide layer 102 and a pad nitride layer 104 are sequentially formed onthe first top surface S1 of the substrate 100. In some embodiments, thepad oxide layer 102 includes silicon oxide (SiO₂) and the pad nitridelayer 104 includes silicon nitride (Si₃N₄). It should be understood thatthe pad oxide layer 102 and pad nitride layer 104 may be replaced withother suitable materials that provide high etching selectivity withrespect to the substrate 100. In some embodiments, the pad oxide layer102 can be deposited by conventional depositional processes, such as achemical vapor deposition (CVD) process, or can be formed by thermallyoxidizing a top thin portion of the substrate 100 in a furnace. The padoxide layer 102 may be used to reduce an interfacial stress between thesubstrate 100 and the subsequently-formed pad nitride layer 104. In someembodiments, the pad nitride layer 104 is formed using a low-pressurechemical vapor deposition (LPCVD) process or a plasma-enhanced chemicalvapor deposition (PECVD) process. The pad nitride layer 104 may be usedas a barrier layer against water or oxygen molecules diffusing into thesubstrate.

Next, referring to FIG. 6, a first photoresist pattern 106 is formed onthe pad nitride layer 104 to define a location of an isolation region.In some embodiments, the first photoresist pattern 106 includes multiplefirst openings O1 exposing top surfaces of the pad nitride layer 104.Specifically, the formation of the first photoresist pattern 106 atleast includes sequentially coating a first photoresist layer (notshown) on the pad nitride layer 104, exposing the first photoresistlayer to a radiation using a first photomask (not shown) and alithography process (not shown) and developing the exposed firstphotoresist layer.

Next, referring to FIG. 7, the substrate 100, the pad oxide layer 102and the pad nitride layer 104 are etched using the first photoresistpattern 106 as an etching mask. Specifically, portions of the substrate100, the pad oxide layer 102 and the pad nitride layer 104 exposed bythe first openings O1 are removed. Therefore, an isolation trench T1 isformed in the substrate 100 and the first photoresist pattern 106 isthen removed using an ashing process or a wet strip process.

Subsequently, referring to FIG. 8, the pad oxide layer 102 and the padnitride layer 104 exposing the isolation trench T1 are removed using awet strip process. At such time, the first top surface S1 of thesubstrate 100 is exposed again.

Next, referring to FIG. 9, the isolation trench T1 is filled with afirst dielectric material 110 using a CVD process or a spin-coatingprocess. In some embodiments, the first dielectric material 110 includesat least one of, silicon oxide (SiO₂), tetraethyl orthosilicate (TEOS),boron phosphorus silicate glass (BPSG) and undoped silicate glass (USG).In some embodiments, after the isolation trench T1 is filled with thefirst dielectric material 110, a chemical mechanical planarization (CMP)process may be performed to planarize a top surface of the firstdielectric material 110 so that the top surface of the first dielectricmaterial 110 does not protrude above the first top surface S1 of thesubstrate 100.

Still referring to FIG. 9, the isolation trench T1 filled with the firstdielectric material 110 form an isolation region BB. In someembodiments, the isolation region BB may be arranged at predeterminedintervals in the substrate 100. In addition, an active region AA issurrounded by the isolation region BB and multiple active regions AA maybe alternately arranged with the isolation region BB in the substrate100.

With reference to FIG. 10, an ion implantation process is performed onthe substrate 100 according to step S105 in FIG. 3. Specifically, theion implantation process may include one or more doping processes. Forexample, a dopant 112 may be implanted into the substrate 100 to form animpurity region 114 in the active region AA. The impurity region 114 issurrounded by the isolation region BB and the bottom surface of theimpurity region 114 may be positioned at a predetermined depth from thefirst top surface S1 of the substrate 100. In some embodiments, theimpurity region 114 can be an n-type doped region when the dopant 112includes phosphorus (P) or arsenic (As). At such time, the impurityregion 114 has electrons as the majority carrier. In some embodiments,the impurity region 114 can be a p-type doped region when the dopant 112includes boron (B), gallium (Ga) or indium (In). At such time, theimpurity region 114 has electron holes as the majority carrier. In someembodiments, an annealing process may be performed to repair the damagecaused by the ion implantation process and to activate the dopant 112.

With reference to FIG. 11 and FIG. 12, a recess formation process isperformed on the substrate 100 according to step S107 in FIG. 3. First,referring to FIG. 11, a second photoresist pattern 120 is formed on thesubstrate 100 to define the location of recesses. In some embodiments,the second photoresist pattern 120 includes multiple second openings O2exposing a top surface of the impurity region 114. Specifically, theformation of the second photoresist pattern 120 at least includessequentially coating a second photoresist layer (not shown) on theactive region AA and the isolation region BB, exposing the secondphotoresist layer to a radiation using a second photomask (not shown)and a lithography process (not shown) and developing the exposed secondphotoresist layer.

Next, referring to FIG. 12, the active region AA is etched using thesecond photoresist pattern 120 as an etching mask. Specifically,portions of the active region AA exposed by the second openings O2 areremoved. Therefore, multiple recesses T2 are formed in the active regionAA and the second photoresist pattern 120 is then removed using anashing process or a wet strip process. In some embodiments, the depth ofthe recess T2 is less than that of the isolation trench T1. In someembodiments, the recess T2 is a line-type channel that extends in anyone direction in the active region AA. Therefore, the impurity region114 is divided by the recesses T2 into multiple impurity regions 114. Insome embodiments, bottom surfaces of the impurity regions 114 are higherthan the bottom surfaces of the recesses T2.

With reference to FIG. 13, an insulating liner 130 is formed on thesubstrate 100 according to step S109 in FIG. 3. Specifically, first, theinsulating liner 130 is deposited over the active region AA and theisolation region BB and conformally formed within the recesses T2. Next,a CMP process is performed to remove portions of the insulating liner130 over the first top surface S1. As a result, remaining portions ofthe insulating liner 130 line the inner sidewalls of the recesses T2. Insome embodiments, the insulating liner 130 is formed using a CVDprocess. Preferably, the insulating liner 130 is formed using an atomiclayer deposition (ALD) process to allow for formation of a highlyconformal insulating liner 130 having a more uniform thickness. In someembodiments, the insulating liner 130 includes silicon oxide (SiO₂) orother suitable materials.

With reference to FIG. 14, a first conductive material 140 is formed onthe substrate 100 according to step S111 in FIG. 3. Specifically, first,the first conductive material 140 is deposited over the active region AAand the isolation region BB and completely fills the recesses T2 linedwith the insulating liner 130. Next, a CMP process is performed toremove the first conductive material 140 over the first top surface S1.As a result, the first conductive material 140 surrounded by theinsulating liner 130 is left in the recesses T2.

In some embodiments, the first conductive material 140 is formed using aCVD process, a physical vapor deposition (PVD) process or anelectroplating process. In some embodiments, the first conductivematerial 140 includes various metals such as aluminum (Al), copper (Cu),tungsten (W), titanium (Ti) or other suitable materials. In someembodiments, before the first conductive material 140 is deposited onthe insulating liner 130, a metal seed layer (not shown) is conformallyformed on the insulating liner 130 to assist in the adhesion between theinsulating liner 130 and the subsequently-formed first conductivematerial 140. The material of the metal seed layer is selected accordingto the material used in the first conductive material 140.

Still referring to FIG. 14, in some embodiments, the first conductivematerial 140 surrounded by the insulating liner 130 in the substrate 100forms a signal line or a power line. Specifically, the first conductivematerial 140 located at the central portion of the active region AAforms a buried power line BPL and the first conductive material 140located at the peripheral portion of the active region AA forms a buriedsignal line BSL. In other words, the buried power line BPL is distal tothe isolation region BB and the buried signal line BSL is proximal tothe isolation region BB. In some embodiments, the buried power line BPLand the buried signal line BSL extend in a first direction D1, as shownin FIG. 14. In addition, multiple buried power lines BPL and multipleburied signal lines BSL are arranged along a second direction D2orthogonal to the first direction D1. The buried power line BPL may beprovided with a supply voltage (V_(cc)) to power the electroniccomponents that will be subsequently formed thereon. The buried signalline BSL may be electrically coupled to multiple signal-transmissionpins (not shown) used to transmit various data signals (DQ) or datastrobe signals (DQS).

With reference to FIG. 15, a first circuit layer 150 and a secondcircuit layer 152 are formed on the substrate 100 according to step S113in FIG. 3. Specifically, the first circuit layer 150 and the secondcircuit layer 152 are disposed on the first top surface S1 of thesubstrate 100 and separated from each other by a predetermined distancew1. In some embodiments, the first circuit layer 150 covers the buriedpower lines BPL and the buried signal lines BSL, and the first circuitlayer 150 is electrically coupled to the buried power lines BPL and theburied signal lines BSL. The second circuit layer 152 does not cover theburied power lines BPL or the buried signal lines BSL. In someembodiments, the first circuit layer 150 and the second circuit layer152 may function as a sense amplifier (SA) circuit or a sub-word linedriver (SWD) circuit. The first circuit layer 150 and the second circuitlayer 152 will be electrically coupled to the electronic componentssubsequently formed thereon.

With reference to FIG. 16, multiple cell capacitors 170 are formed overthe substrate 100 according to step S115 in FIG. 3. Specifically, thecell capacitors 170 are electrically coupled to the first circuit layer150 via multiple landing pads 160, wherein one cell capacitor 170 isdisposed on each landing pad 160 formed on the first circuit layer 150.In addition, the cell capacitor 170 is electrically coupled to theimpurity region 114. The cell capacitor 170 is used to store a charge,which represents a bit of information. The formation of the landing pad160 may include at least a lithographic process, an etching process anda deposition process known in the art. In some embodiments, the materialof the landing pad 160 includes tungsten (W), copper (Cu), aluminum (Al)or alloys thereof, but is not limited thereto.

It should be understood that the cell capacitor 170 shown in FIG. 16 isfor illustration purpose only and the detailed architecture of the cellcapacitor 170 is not shown. In some embodiments, the cell capacitor 170at least includes a bottom electrode, a top electrode and a capacitordielectric material encased by the bottom electrode and the topelectrode. The bottom electrode and the top electrode may be aconductor, such as a metal, alloys or polysilicon. The capacitordielectric material may be formed with one or more high-k dielectricmaterials, such as hafnium oxide (HfO₂), zirconium oxide (ZrO₂),tantalum oxide (Ta₂O₅), aluminum oxide (Al₂O₃) or the like. In someembodiments, the cell capacitor 170 may be any shape of capacitors knownin the art. For example, the shape of the cell capacitor 170 can besimple, such as a rectangle, or complex, such as concentric cylinders orstacked discs.

With reference to FIG. 17, multiple word line structures 190 are formedover the substrate 100 according to step S117 in FIG. 3. Specifically,the word line structures 190 are electrically coupled to the cellcapacitors 170 via first conductive plugs 180, wherein each word linestructure 190 is disposed on a first conductive plug 180 formed on acell capacitor 170. In some embodiments, the cell capacitor 170 isinterposed between the first circuit layer 150 and the word linestructure 190.

The formation of the first conductive plug 180 may include at least alithographic process, an etching process and a deposition process knownin the art. In some embodiments, the first conductive plug 180 is aconductor, such as a metal, alloys or polysilicon. It should beunderstood that the word line structure 190 shown in FIG. 17 is forillustration purpose only and the detailed architecture of the word linestructure 190 is not shown.

In some embodiments, the word line structure 190 at least includes agate dielectric material, a gate electrode and a gate spacer. The gateelectrode is disposed on the gate dielectric material and the gatespacer. The gate dielectric material is surrounded by the gate spacer.In some embodiments, the gate dielectric material includes silicon oxide(SiO₂) or other suitable materials. In some embodiments, the gateelectrode is a metal gate that includes tungsten (W), aluminum (Al),copper (Cu), titanium (Ti) or other materials with a proper workfunction or a polysilicon gate.

In some embodiments, the gate spacer is an insulator that may includenitride, low-k dielectrics or other suitable materials. In someembodiments, the word line structure 190 may include a non-silicon basedmaterial or a heat-sensitive material. In some embodiments, the wordline structure 190 extends in the first direction D1, as shown in FIG.17. In addition, multiple word line structures 190 are arranged alongthe second direction D2 orthogonal to the first direction D1.

With reference to FIG. 18, multiple second conductive plugs 200 areformed on the word line structures 190 according to step S119 in FIG. 3.Specifically, each second conductive plug 200 is disposed on a word linestructure 190. The formation of the second conductive plug 200 mayinclude at least a lithographic process, an etching process and adeposition process known in the art. In some embodiments, the secondconductive plug 200 is a conductor, such as a metal, alloys orpolysilicon.

In some embodiments, stacks of the landing pad 160, the cell capacitor170, the first conductive plug 180, the word line structure 190 and thesecond conductive plug 200 extend along a third direction D3substantially orthogonal to both the first direction D1 and the seconddirection D2. In some embodiments, the gate electrode in the word linestructure 190 may serve as the gate terminal of a cell transistor usedto control the word line structure 190. The first conductive plug 180and the second conductive plug 200, which are immediately below andabove the word line structure 190, may serve as a source terminal and adrain terminal of the cell transistor. The cell transistor acts as aswitch for the cell capacitor 170. That is, the cell transistor controlscharging and discharging of the cell capacitor 170.

With reference to FIG. 19, an interlayer dielectric 210 is formed overthe substrate 100 according to step S121 in FIG. 3. Specifically, theinterlayer dielectric 210 covers the first circuit layer 150, the secondcircuit layer 152 and a portion of the impurity region 114. In addition,the interlayer dielectric 210 encapsulates the landing pads 160, thecell capacitors 170, the first conductive plugs 180, the word linestructures 190 and the second conductive plugs 200.

In some embodiments, the interlayer dielectric 210 mainly includes oxidesuch as silicon oxide (SiO₂) or other suitable materials formed using aCVD process. In some embodiments, the formation of the interlayerdielectric 210 may include several steps. For example, in a first step,the interlayer dielectric 210 may be deposited to a level that is evenwith the top surface of the cell capacitor 170. In a second step, theinterlayer dielectric 210 may be deposited to a level that is even withthe top surface of the word line structure 190. In a third step, theinterlayer dielectric 210 may be deposited to completely cover thesecond conductive plug 200. After the third step, a CMP process isperformed to planarize the interlayer dielectric 210 to expose the topsurface of the second conductive plug 200. At such time, the interlayerdielectric 210 has a planar second top surface S2 coplanar with the topsurface of the second conductive plug 200.

With reference to FIG. 20, multiple through holes T3 are formed topenetrate the interlayer dielectric 210 according to step S123 in FIG.3. Specifically, the formation of the through holes T3 at least includesforming a photoresist pattern (not shown) on the interlayer dielectric210, etching the interlayer dielectric 210 until the second circuitlayer 152 is exposed and then removing the photoresist pattern.

With reference to FIG. 21, a second conductive material 220 is formed onthe substrate 100 according to step S125 in FIG. 3. Specifically, first,the second conductive material 220 is deposited over the interlayerdielectric 210 and completely fills the through hole T3. Next, a CMPprocess is performed to remove the second conductive material 220 overthe second top surface S2. As a result, the second conductive material220 is left in the through hole T3 surrounded by the interlayerdielectric 210. In some embodiments, the second conductive material 220is formed using a CVD process, a PVD process or an electroplatingprocess. In some embodiments, the second conductive material 220includes various metals such as aluminum (Al), copper (Cu), tungsten(W), titanium (Ti) or other suitable materials.

With reference to FIG. 22, a bit line structure 230 is formed on theinterlayer dielectric 210 according to step S127 in FIG. 3.Specifically, the bit line structure 230 is disposed over the word linestructure 190. In some embodiments, the word line structure 190 isinterposed between the cell capacitor 170 and the bit line structure230. In some embodiments, the bit line structure 230 is formed using aCVD process, a PVD process or an electroplating process. In someembodiments, the bit line structure 230 is a conductor, such as a metalor polysilicon. Preferably, the bit line structure 230 is a metal alloy,such as tungsten silicide (WSi). In some embodiments, the bit linestructure 230 extends in the second direction D2, as shown in FIG. 22.At such time, a semiconductor structure ST1 is generally formed, whereinthe semiconductor structure ST1 primarily includes a memory array.

In some embodiments, the bit line structure 230 is electrically coupledto the word line structure 190 and the cell capacitor 170. The bit linestructure 230 may be used to transmit a signal to the cell capacitor 170so that data stored in the cell capacitor 170 can be read, or the signalcan be stored as data and written in the cell capacitor 170. In someembodiments, the second conductive material 220 deposited in the throughhole T3 is substantially a bit line contact (BLC) electricallyconnecting the bit line structure 230 to the second circuit layer 152and to the impurity region 114. In some embodiments, the bit linecontact extends in the third direction D3.

FIG. 23 is a schematic top view of the semiconductor structure ST1 inFIG. 22, in accordance with some embodiments of the present disclosure.Referring to FIG. 23, the buried power line BPL, the buried signal lineBSL and the word line structure 190 extend in the first direction D1 andthe bit line structure 230 extends in the second direction D2substantially orthogonal to the first direction D1. Therefore, multipleword line structures 190 and multiple bit line structures 230 may formthe columns and rows of a memory array. Compared with the buried powerline BPL and the buried signal line BSL within the substrate 100, theword line structure 190 is discrete from the substrate 100 such that theword line structure 190 is spatially higher than both the buried powerline BPL and the buried signal line BSL.

However, the word line structure 190, the buried power line BPL and theburied signal line BSL are basically arranged along the second directionD2. The bit line structure 230 is more discrete from the substrate 100than the word line structure 190 such that the bit line structure 230 isspatially higher than the word line structure 190. A cell capacitor 170not shown in FIG. 23 is located at the intersection of each word linestructure 190 and each bit line structure 230. In some embodiments, theword line structures 190 and the bit line structures 230 substantiallyform a 4F² layout for a memory array.

In the present disclosure, power lines and signal lines are buried inthe substrate while main components of a memory array, i.e., word linestructures, bit line structures and cell capacitors, are disposed overthe substrate. In addition, a capacitor-first process is adopted forfabricating the memory array with a 4F² layout when heat-sensitivenon-silicon based materials are used in the fabrication of celltransistors. Due to the minimized area of unit memory cells, spaceavailable for metal routing including the arrangement of power lines andsignal lines adjacent to the main components becomes limited. Thepresent application employs multiple recesses that are generally used toaccommodate buried word line structures in a 6F² layout for a memoryarray to accommodate power lines and signal lines. The advantage ofdisposing the power line and the signal line in the recess includessaving the space originally used for metal routing above the substrate.As a result, there is no need to reserve space adjacent to the celltransistor or the cell capacitor for the metal routing. In addition, thearrangement of the word line structure, the bit line structure or thecell capacitor disposed over the substrate can be optimally adjusted dueto the extra space.

Although the present disclosure and its advantages have been describedin detail, it should be understood that various changes, substitutionsand alterations can be made herein without departing from the spirit andscope of the disclosure as defined by the appended claims. For example,many of the processes discussed above can be implemented in differentmethodologies and replaced by other processes, or a combination thereof.

Moreover, the scope of the present application is not intended to belimited to the particular embodiments of the process, machine,manufacture, and composition of matter, means, methods and stepsdescribed in the specification. As one of ordinary skill in the art willreadily appreciate from the present disclosure, processes, machines,manufacture, compositions of matter, means, methods or steps, presentlyexisting or later to be developed, that perform substantially the samefunction or achieve substantially the same result as the correspondingembodiments described herein, may be utilized according to the presentdisclosure. Accordingly, the appended claims are intended to includewithin their scope such processes, machines, manufacture, compositionsof matter, means, methods and steps.

What is claimed is:
 1. A semiconductor structure, comprising: asubstrate having a first top surface; an active region surrounded by anisolation region in the substrate; a buried power line and a buriedsignal line disposed within the substrate and in the active region; afirst circuit layer disposed on the first top surface of the substrateand covering the buried power line and the buried signal line; a secondcircuit layer disposed on the first top surface of the substrate andseparated from the first circuit layer; and a cell capacitor disposed onand electrically coupled to the first circuit layer.
 2. Thesemiconductor structure according to claim 1, wherein the buried powerline is disposed at a central portion of the active region, and theburied signal line is disposed at a peripheral portion of the activeregion.
 3. The semiconductor structure according to claim 1, wherein theburied power line is distal to the isolation region, and the buriedsignal line is proximal to the isolation region.
 4. The semiconductorstructure according to claim 1, further comprising a word line structuredisposed over the cell capacitor.
 5. The semiconductor structureaccording to claim 4, further comprising an interlayer dielectricencapsulating the cell capacitor and the word line structure.
 6. Thesemiconductor structure according to claim 5, further comprising asecond conductive material within a through hole penetrating theinterlayer dielectric.
 7. The semiconductor structure according to claim5, further comprising a bit line structure disposed on the interlayerdielectric and over the word line structure.
 8. The semiconductorstructure according to claim 5, wherein the buried power line, theburied signal line and the word line structure extend along a firstdirection, and the bit line structure extends along a second directionsubstantially orthogonal to the first direction.
 9. The semiconductorstructure according to claim 8, wherein the second conductive materialin the through hole extends along a third direction substantiallyorthogonal to the first direction and the second direction.
 10. Thesemiconductor structure according to claim 8, wherein the buried powerline and the buried signal line are arranged along the second direction.11. The semiconductor structure according to claim 10, wherein the wordline structure and the bit line structure form a memory array, whereinthe memory array has a layout of four square feature size (4F²).
 12. Thesemiconductor structure according to claim 10, wherein the cellcapacitor is interposed between the first circuit layer and the wordline structure, and the word line structure is interposed between thecell capacitor and the bit line structure.